1. Field of the Invention
This invention relates to a generator system for a motor vehicle.
2. Description of Related Art
Motor vehicles are normally provided with an ac generator (alternator) to supply power for energizing lamps, motors, and various actuators. Such a generator is usually provided with a field winding and an armature winding, the field winding thereof being rotated by the engine to induce a rotating magnetic field and the armature winding thereby producing power having a comparatively low voltage of approximately 12 V.
Presently, however, the demand for an output voltage substantially higher than the present 12 V of the known generator is increasing, for instance, because of the requirement for use of commercial line home appliances on board a motor vehicle, and because of the increased number of components requiring high power, such as electrically heated windshields and seats. Such high power requirements also require large diameter cable which is inconvenient. As an example, twenty years ago a 35A, 12 V output generator was normal, but it is expected that by 1995 a generator having a 150A output will be required. Usually about 2 V is dropped across the rectifier stack in an ac generator, which means that in a 12 V system producing 150A, the power drop is 150.times.2=300 W whereas if the system voltage is increased to 48 V, for example, the power drop across the rectifier stack is 12/48.times.150.times.2=75 W, i.e. a significant power saving and consequent reduction in heat and generator connecting cable size. It is, therefore, proposed to increase the supply voltage instead of the current capacity to energize the increasing number of car electronic components. A means for generating and supplying a voltage higher than the conventional generator 12 V by adding another generator is disclosed in Japan Patent Laid-Open No. 55-53152 (1980).
This prior art, however, has not paid attention to a possible danger that might be attendant with a high tension voltage. In this respect, for the sake of simplification of circuitry, one end of the output terminals of the generator is usually at ground potential. Because of such a design, even if a body part of a passenger inadvertently touches a connecting wire of the generator which is at other than ground potential, there is the danger that the whole voltage in the wire is likely to discharge through the body, thus jeopardizing passenger safety. It is an object of this invention to provide a motor vehicle generator system having enhanced safety.